Voltage comparators are useful circuit blocks that can be implemented to operate in a wide variety of applications. For example, voltage comparators may be utilized to detect zero crossings of an arbitrary signal waveform. As another example, voltage comparators may be utilized to convert sine waves into square waves. Voltage comparators can also be used in current sensing applications by monitoring the voltage at a specified node of a circuit.
A block diagram of a conventional voltage comparator 10 is shown in FIG. 1. The voltage comparator includes inputs 12 and 14 and an output 16. The input 12 is configured to receive an input voltage V.sub.i, while the input 14 is configured to receive a reference voltage V.sub.r. The reference voltage V.sub.r constitutes the comparator threshold. In operation, if the input voltage V.sub.i is greater than the reference voltage V.sub.r, the voltage comparator provides a high level signal V.sub.o+ on the output. Alternatively, if the input voltage V.sub.i is less than the reference voltage, the voltage comparator provides a low level signal V.sub.o- on the output.
The transfer characteristics of the voltage comparator 10 are illustrated in FIG. 2. The plot 18 of FIG. 2 is the voltage on the output 16 of the voltage comparator with changes to the input voltage V.sub.i When V.sub.i &lt;V.sub.r, the voltage on the output is the low level signal V.sub.o-. However, when V.sub.i&gt;V.sub.r, the voltage on the output is the high level signal V.sub.o+. For a zero crossing detection application, the reference voltage V.sub.r equals zero voltage. In other applications, however, the reference voltages can be other than zero voltage. For a wave conversion application, the high level signal V.sub.o+ and the low level signal V.sub.o- are selectively set to produce the desired square wave.
Conventional voltage comparators, as illustrated by the voltage comparator 10 of FIG. 1, may operate well for their intended purposes. However, when the input voltage to be compared is very close to the reference voltage (e.g., supply voltage or the electrical ground), these conventional voltage comparators may not function as designed. In light of this concern, what is needed is a voltage comparator that can operate properly even when the input voltage is in the order of millivolts with respect to the reference voltage, such as the supply voltage or the electrical ground.